Select a ZIP archive and hit Space. Finder will 'helpfully' tell you its size and entirely ignore what's inside it. Install BetterZip Quick Look Generator and you'll instead be able to peek inside any ZIP archive, even expanding and collapsing the folders within. It's one of the simplest add-ons here, but if you work with a lot of archives, it's a huge-time-saver.
Free, Kevin Gessner
Media keys on Apple keyboards are useful, providing fast access to media playback controls, Launchpad, Mission Control and brightness settings. However, Apple hardwires controls to keyboards, meaning if you've an old Mac running a new OS, you're likely lumbered with a pointless Dashboard key. System Preferences enables you to turn all these functions off, but FunctionFlip is far more useful, because you can 'flip' individual keys. This means with an old Mac, you can, for example, leave the media keys on, 'flip' just F4 and then bind that key to Launchpad in the Keyboard System Preferences pane.
Free, Lighthead Software
If your screensaver or sleep mode kicks in after a short amount of time, this can be an inconvenience when you're doing something on your Mac that doesn't require interaction, such as reading. Caffeine simply keeps your Mac awake when it's active, a status denoted rather sweetly by a steaming coffee cup in the menu bar. Command-click and you also get access to a number of timer options other than 'indefinite'.
£13 ($19.95), Ecamm
Apple's iOS devices merrily print to any printer that's AirPrint compatible. These days, that's quite a few, but many people still have older printers they don't want rid of. Printopia acts as a bridge between shiny new iOS devices and veteran printer kit, and it also provides options to print jobs directly to your Mac. The $19.95 price-tag means it isn't the cheapest Mac add-on, but it's a lot more affordable than a new printer!
- Now why not check out OS X Mavericks release date tipped for late October